Glucocorticoids are an effective treatment for inflammatory disease such as asthma and rheumatoid arthritis. However severe systemic side effects limit the dose that can be given and their long-term utility. The side effects include suppression of the hypothalamic-pituitary-adrenal (HPA) axis, osteoporosis, reduced bone growth in the young, behavioral alterations and disorders in lipid and glucose metabolism.
The glucocorticoid receptor (GR) is a member of a gene family known as the nuclear hormone receptors. After binding their cognate ligand, these receptors are activated and are capable of regulating transcription both positively and negatively. The detailed mechanism of this regulation, though not entirely understood, has become increasingly clear (R. M. Evans, Science, 1988, 240: 889-895; R. H. Oakley and J. Cidlowski, Glucocorticoids, N. J. Goulding and R. J. Flowers (eds), Boston: Birkhauser, 2001, 55-80). Glucocorticoids can freely diffuse across the plasma membranes into the cell where they bind to GR present within the cytoplasm. Once bound, a conformational change in the receptor causes the release of several chaperone proteins allowing the GR/ligand complex to translocate to the nucleus, dimerize and bind specifically and tightly to palindromic DNA sequences in the promoters of regulated genes. Hormone-bound receptor then recruits a group of proteins known as the coactivator complex. This complex is required to initiate transcription, and works by recruiting both the transcriptional machinery of the cell and histone acetyltransferases involved in opening the chromatin in the vicinity of the promoter. The transcription of a number of genes that contain GREs (glucocorticoid response elements) in their promoters is activated by GR. These include genes involved in gluconeogenesis, intermediary metabolism and the stress response.
In addition to transcriptional control exerted by GR at GREs, numerous genes, particularly those involved in the inflammation response, must be controlled through alternative mechanisms, since no GREs appear in the promoters of these genes. The promoters of numerous pro-inflammatory genes do contain binding sites for the transcription factors NF-kB and AP-1. It has been shown that the GR/ligand complex represses transcription of the pro-inflammatory genes by directly interacting with NF-kB or AP-1 and preventing transcriptional upregulation by the transcription factors (C. Jonat et al., Cell, 1990, 62: 1189-1204; H. F. Yang-Yen et al., Cell, 1990, 62: 1205-1215; A. Ray and K. E. Prefontaine, Proc. Natl. Acad. Sci. U.S.A., 1994, 91: 752-756). In vitro work with GR mutants incapable of DNA binding demonstrated that transrepression mediated by GR could be genetically dissociated from transactivation (S. Heck et al., EMBO J., 1994, 17: 4087-4095). This dissociation is further supported by a study where ‘knock-in’ transgenic mice were generated in which wild-type GR was substituted with a similar DNA binding domain mutant (H. M. Reichardt et al., Cell, 1998, 93: 531-541). These mice were incapable of regulating GRE-dependent GR target genes such as tyrosine amino transferase (TAT) or genes that are negatively regulated through interaction with a negative GRE, such as pro-opiomelanocortin (POMC). In contrast, these mice are capable of transrepressing genes activated by NF-kB or AP-1. Thus, the currently accepted model for corticosteroid control of inflammation predicts that GR, NFkB and AP-1 interact in a complex regulatory network leading to repression of cytokine expression.
According to this model a glucocorticoid modulator that would retain the transrepression activity and lose the transactivation activity would have fewer of the side effects associated with adrenal suppression, behavioral alterations, and gluconeogenesis. The anti-inflammatory affects would be retained.